You basically exchange coins for a sub-resource at an exchange rate of 3:5.
What if you skipped the proxy and when you use rations (such as when you take the Undertake a perilous journey or Make Camp move) you pay coins directly to represent the cost of rations?
That way you don’t need to manage a sub-resource, you just pay a direct cost.
You imply at the table that when you find coins you sometimes also find stuff to eat and/or you forage as you travel and resupply whenever you can.
For sake of simplicity, 1 coin / ration.